1. Field of the Invention
The present invention relates generally to the field of control systems for treatment to reduce water alkalinity. More specifically, the present invention discloses a control system to regulate the addition of lime to reduce water alkalinity by continuously monitoring the conductivity of the water and its rate of change.
2. Statement of the Problem
Lime has long been used to treat water to reduce hardness or alkalinity. Lime reacts with bicarbonates and magnesium salts in the water to form insoluble calcium carbonate and magnesium salts which precipitate from solution. It has also been recognized that the conductivity of water initially progressively decreases to a minimum value with addition of lime during precipitation of bicarbonate alkalinity. After essentially complete precipitation of the bicarbonates has occurred, conductivity then begins to increase with further addition of lime. This results in a U-shaped conductivity curve as shown in FIG. 2.
Optimal control of the rate at which lime is added is important for a number of reasons. First, optimal control ensures consistent quality of the treated water. Second, it minimizes the amount of lime necessary for treatment and therefore reduces the operating costs involved. Finally, optimal control minimizes the resulting solid waste that must be disposed of.
A number of control systems have been invented in the past to regulate addition of lime or other feed materials for water treatment, including the following:
______________________________________ Inventor Patent No. Issue Date ______________________________________ Gustafson 3,236,128 March 1, 1966 Carlson 3,462,364 Aug. 19, 1969 King 3,474,032 Oct. 21, 1969 Zaander, et al. 3,605,775 Sep. 20, 1971 Lang, et al. 4,170,553 Oct. 9, 1979 Haga, et al. 4,282,093 Aug. 4, 1981 Campbell, et al. 4,544,489 Oct. 1, 1985 Conlan 4,818,412 April 4, 1989 Eberhardt 4,882,072 Nov. 21, 1989 Loftis, et al. 5,023,803 June 11, 1991 ______________________________________
King discloses a system for controlling lime feed to a chemical treating process wherein the conductivity of the solution varies with lime addition. In particular, the King system is based on the U-shaped conductivity curve discussed above. The King system uses a sample cell to periodically determine the minimum conductivity. A correction signal is then generated to adjust the feed setting to maintain a desired set point based upon the difference between this minimum conductivity and the current conductivity of a sample taken from the treating unit.
Carlson discloses a control system for optimizing the addition of lime for water treatment. A pacer unit determines the conductivity of water at optimal treatment. The conductivity of the water in the treating unit is also measured. The controller calculates the ratio of the conductivities of the pacer unit and the treating unit. The controller adjusts the feed rate of lime into the treating unit to maintain a predetermined conductivity ratio.
Gustarson discloses another example of control system for optimizing the addition of lime for water treatment based upon the conductivity ratio between raw water and treated water.
Zaander, et al., disclose a proportional control system for treatment processes using a variable-capacity feed pump.
The patents to Lang, et al., and Haga, et al., disclose additional examples of treatment systems that use continuous sampling of a separate stream or a pacer cell.
Campbell, et al., disclose a control process to minimize the amount of polymer added to sewage sludge for the purpose of dewatering. The controller seeks the maximum shear stress of the resulting mixture and monitors the slope of this curve to determine a peak.
Conlan discloses a system for controlling the feed rate of hypochlorite solution for removing impurities from water in a water treatment system. The oxidation/reduction potential of the effluent is continuously measured and used to control the feed rate of hypochlorite solution to maintain a predetermined oxidation/reduction potential as a set point.
Eberhardt discloses a vessel for treating bodies of water, such as lakes, to correct chemical, biological, or other imbalances in the aquatic environment. The system continuously adjusts the rate at which the treatment agent is dispensed to account for the speed of the boat and the depth of the water. The pH of the water can also be monitored.
Loftis, et al., disclose a process to control addition of sodium carbonate to precipitate calcium impurities from brine. The concentrations of calcium and carbonate ions are monitored by an automatic titration system. This data is used by a microprocessor to determine the addition rate of sodium carbonate (col. 3, line 39, et seq.).
3. Solution to the Problem
None of the prior art references uncovered in the search show a control system to regulate addition of lime for water treatment in which the feed rate is controlled by both the conductivity ratio and the rate of change of the conductivity ratio.